1. Field of Invention
The present invention relates to a light source current sensing circuit and driving circuit in a display device. More particularly, the present invention relates to a light source current sensing circuit and driving circuit in a display device, for improving efficiency.
2. Description of Related Art
Light emitting diodes (“LEDs”) are known act as a source of emitted light for a wide variety of applications. LEDs are known to provide many advantages over incandescent and fluorescent illumination because of their long operating life, high efficiency, light weight, and low profile. LED light output is proportional to current therethrough. Problems can be particularly egregious for portable devices where battery output gradually decreases with time of use. Therefore, an LED driver circuit is needed that is relatively immune to small voltage fluctuations.
Moreover, LEDs are particularly convenient for applications such as backlighting liquid crystal display (“LCD”) screens. The LEDs are often configured as a set of serial connected LEDs, sometimes referred to in the art and hereinafter as “LED chains,” “LED sets,” “LED banks,” or the like. Lightweight, battery-powered devices, such as mobile computing and communications devices—e.g., personal digital assistant, cellular telephone, electronic book, and the like—may use LEDs as a back light or side light. It is important to sense the current provided to LEDs to provide adequate lighting and to minimize flickering on the screen.
FIG. 1 shows a conventional circuit diagram for sensing current provided to LEDs. As shown in FIG. 1, the current sensing circuit includes a transistor T11 and a reference resistor Rref1. For sensing a current I1 through an LED bank 101 including LEDs 101-1˜101-n, a voltage drop Vsen1 across the reference resistor Rref1 is detected. The detected voltage Vsen1 is fed to a control loop (not shown) for comparing with a reference voltage and the comparison result is used for adjusting/determining the voltage Vboost provided to the LED bank 101. Further, in order to cut off the current I1 to the resistor Rref1, the transistor T11 is provided. The transistor T11 is turned on or off according to a control signal C1. For example, when the transistor T11 is an NMOS transistor, it is turned ON if the control signal C1 is logic HIGH and OFF if logic LOW.
In order to reduce power consumption by the resistor Rref1, the resistance value of the resistor Rref1 is preferred to be small. However, as known, it is not easy to precisely control the resistance of a small resistor. Besides, the addition of the transistor T11 also reduces the efficiency of the control loop.
Therefore, a current sensing circuit for sensing a current flowing through the LED bank(s) and a driving circuit for driving the LED bank(s) are needed, which may increase the efficiency and eliminate the disadvantages occurred in controlling small reference resistors.